Polarization imaging systems have typically been complex, expensive, and unsuitable for applications in harsh environments. The technique of Polarization Difference Imaging (PDI) requires that two or more images must be taken (sensing different or orthogonal planes of polarization within a stimulus), and the spatial difference in intensity of the recorded images be determined by the subtraction of one image from the other.
Typically PDI systems have either utilized mechanically rotated optical polarizers, or more recently, electrically tunable liquid crystal polarizers. In both cases, the resultant “difference image” includes the amount of polarized light that is reflected directly off the surface of the object, and (because of common mode rejection of randomly polarized light) the veiling effects of scattering caused by intervening medium such as smoke, fog, or suspended particulates in fluids is subtracted.
It has been demonstrated that even if less than 1 percent of the returning light from the target is polarized, a PDI image can be made which can reveal even subtle surface details and textures. PDI therefore can greatly enhance target detection and by substantially enhancing contrast and reducing noise in the presence of intervening scattering medium or mechanisms. For example, in underwater imaging, PDI has been shown to extend the distance at which objects can be imaged by a factor of three.
Common to all existing PDI systems is the requirement to take two distinct polarization images separated in time. As a result, creating a “real-time” PDI video system has been problematic.